The present invention generally relates to molding equipment and more particularly, to extrusion molding equipment for molding a multi-layer parison having a plurality of resin layers laminated therein by the use of an extrustion molding technique.
Conventionally, although a hollow article of a resin material is generally formed through a blow molding process in which a pressurized gas is blown into a parison inserted into a set of molds, a multi-layer parison having a plurality of resin layers laminated therein has been occasionally employed in the blow molding process so as to complement insufficient properties for a molded product of a single resin material.
In case of a fuel tank made of a resin material such as high-density polyethylene, for example, since there has been a problem of a gasoline permeation in which the gasoline stored within the tank permeates the outside therethrough, the Japalnese Patent Laid-Open Application (Tokkaisho) No. 58-220738 discloses the hollow article having a plurality of resin layers laminated therein and formed from the parison by the use of the blow molding technique so as to deal with this kind of problem. More specifically, the aforementioned tank is formed, through the blow molding process, from the multi-layer parison in which a nylon resin layer for preventing the gasoline from permeating therethrough is so bonded with an inner resin layer and an outer resin layer of the high-density polyethylene as to be securely held therebetween through each of adhesive layers and accordingly, a wall of the tank is constructed by five resin layers of three kinds of resin materials in the order of a high-density polyethylene resin layer, an adhesive layer, a nylon resin layer, an adhesive layer and a high-density polyethylene resin layer in a direction from inside towards outside of the tank wall.
On the other hand, one of known extrusion molding equipment for molding the multi-layer parison as described above is disclosed, for example, in the Japanese Patent Publication (Tokkosho) No. 52-37026 in which there are formed a plurality of cylindrical resin storing spaces by arranging a cylindrical member having a temperature control means therein within a molding head in a layered state and not only a plurality of extruders are so disposed as to communicate respective resin storing spaces for extruding the resin materials thereinto, but also a plurality of ringed pistons are also disposed within respective resin storing spaces for extruding the resin materials therefrom.
The Japanese Patent Laid-Open Application (Tokkaisho) No. 60-187513 discloses a multi-layer preform having an inner layer and an outer layer of a chief resin material, and an intermediate layer composed of a gas barrier resin layer and disposed between the inner and outer layers of the chief resin material. Although the multi-layer preform includes a plurality of the aforementioned layers approximately constant in total thickness, the gas barrier resin layer is caused to vary in thickness in a lengthwise direction of the multi-layer preform. This kind of the multi-layer preform has been generally formed by such an arrangement that a ringed member communicating an injection unit for forcibly extruding the gas barrier resin injected from the injection unit is disposed in an annular resin passage for the chief resin material.
In the conventional extrusion molding equipment as described above, when the multi-layer parison having the five layered construction of three kinds of the resin materials is formed in a manner that the resin layers of auxiliary resin materials composed of the nylon resin layer and the adhesive layers are laminated with each other between the inner and outer layers of the chief resin material corresponding to the high-density polyethylene layers, not only a plurality of the resin storing spaces are necessarily formed for respective inner and outer layers of the chief resin material, but also as many accumulators as the resin storing spaces are required for extruding the resins from the resin storing spaces and accordingly, the extrusion molding equipment has been manufactured disadvantageously in a large size.
Furthermore, although the resin materials stored in respective resin storing spaces are to be controlled in storage temperature by the temperature control means disposed in the cylindrical member, it is substantially impossible to steadily control the temperature of the resin materials due to the fact that the resin storing spaces are formed closely and exert a thermal influence to each other, thus resulting in that a defect such as an imperfect bonding between adjacent resin layers or the like has been disadvantageously taken place in the multi-layer parison.
In addition, unless an extrusion system forcibly extruding the chief resin material is provided with one or more ringed pistons, it is substantially impossible to subject the material to an extrusion pressure evenly acting on whole extrusion surface thereof and to obtain a uniform extrusion speed thereof in case of the extrusion of the material, thus undesirably resulting in that it is remarkably difficult to unify a thickness distribution of the inner and outer layers in a circumferential direction thereof or to obtain a required thickness distribution thereof due to a production of the parison having a partiality in thickness distribution.
Even if the extrusion system is provided with one or more ringed pistons, in the case where the extrusion system is caused to communicate the annular resin passage by way of a confluent conduit, as shown in the aforementioned Application No. 60-187513, a stream of the resin material is subjected to be disturbed and accordingly, it is also difficult to obtain the uniform or the required thickness distribution of respective resin layers.